Spinal cord stimulation with interferential current

ABSTRACT

A stimulator and a method for the treatment of intractable pain syndromes by electrical stimulation of the spinal cord is disclosed in which implantable electrodes positioned around a targeted area of the spinal cord transmit an interferential current that has a base medium frequency alternating current between 500 Hz-20 KHz. A digital signal processor generates a sine-wave-like waveform from a pulse generator which after further processing is used to generate at least two circuits for use in producing the interferential current. An effective area of stimulation is controlled by the quantity of electrodes, positioning of the electrodes and electrode cross pattern orientation. Amplitude modulation of electrical circuits created at the electrode placements also augments the effective area of stimulation. The stimulator and method reduce accommodation of the body to the electrical stimulation and provide deeper penetration of the resultant signal.

REFERENCE TO RELATED APPLICATION

The present application claims the benefit of U.S. Provisional Patent Application No. 60/441,326 filed Jan. 22, 2003 whose disclosure is hereby incorporated by reference in its entirety into the present disclosure.

FIELD OF THE INVENTION

The present invention is generally related to spinal cord stimulation and, more particularly, is related to an apparatus and method for the electrical stimulation of the spinal cord using an interferential current pattern for treating chronic pain conditions.

BACKGROUND OF THE INVENTION

Electrical stimulation of the posterior spinal cord, spinal cord stimulation (SCS), has developed into an effective therapeutic tool for treating chronic pain conditions. However, very little is known about the sites of activation or the neural mechanisms evoked by SCS that relieve pain and promote changes in the function of somatic and visceral structures.

Spinal Cord Stimulation is most commonly used for patients with chronic intractable pain syndromes. It has also been useful for treating movement disorders and is occasionally used following head injuries. However, one complication with SCS is that of accommodation or habituation to the stimulation signal. Companies that manufacture spinal stimulation devices have developed complex stimulation programs and devoted chapters on techniques to reduce the problem of accommodation during SCS (Alfano S, Darwin J, Picullel B: Spinal Cord Stimulation, Patient Management Guidelines for Clinicians, Medtronic, Inc.). Accommodation is when the body habituates or becomes accustomed to an activity or signal and then starts to ignore or ‘tune it out’. By varying the signal or keeping the focal point of the signal moving, accommodation can be minimized. The concept of using interferential stimulation with implantable leads to decrease the problem of accommodation might prove to be advantageous.

Dorsal Column Stimulation (DCS) or SCS using an electrical interferential current pattern has shown to be a cost benefit in treating chronic pain disorders in patients (Dorsal column stimulation: cost to benefit analysis; Acta Neurochir Suppl (Wien), 52( ): 121-3, 1991).

SCS stimulates the dorsal column in a somewhat superficial manner as pointed out by Holsheimer (Holsheimer J: Which Neuronal Elements are activated Directly by Spinal Cord Stimulation, Neuromodulation, Volume 5, Number 1: 25-31, 2002). The electrodes are normally attached to the dura matter in the epidural space, and most of the current distribution remains in the cerebrospinal fluid (CSF) and does not project deeply into the dorsal column. Providing an interferential component to the electrode array of the SCS allows the crossing of the two signals wherein the resultant additive effect of the beat frequency produces deeper penetration of the signal and a higher resultant amplitude at the stimulation site. The interferential current would recruit larger numbers of dorsal column fibers and provide greater levels of pain relief and benefit to intractable pain patients.

Thus, a heretofore unaddressed need exists in the industry to address the aforementioned deficiencies and inadequacies with regard to accommodation or habituation to the spinal cord stimulation signal when used in the treatment of chronic pain syndromes.

SUMMARY OF THE INVENTION

Embodiments of the present invention provide an apparatus and method for the treatment of chronic pain syndromes using electrical stimulation of the spinal cord. The present invention utilizes an interferential current that has a base medium frequency alternating current between 500 Hz and 20 KHz. An interferential current is set up between two circuits that are arranged in a cross-pattern on the subject's targeted area of stimulation. Where the circuits superimpose in a cross-pattern, the resultant beat frequency will be the difference between the frequencies of the two circuits and the amplitude will be additive and greater than either circuit alone. The range of the beat frequency is usually between 1-250 Hz. Multiple levels of stimulation can be treated depending upon the electrode placement, pairing and modulation pattern selected. The range of output would be from 0-11 volts per circuit depending on the patient's needs and the pulse width is commonly set at 210 microseconds but it could range from 10-600 microseconds. The amplitude can be modulated in the respective circuits to increase the area of targeted stimulation. This type of current (Interferential) provides improved directional control, decreased accommodation/habituation and increased depth of penetration in comparison to other standard implantable stimulation systems and their accompanying surgical leads. The amplitudes of the outputs in the respective circuits may be modulated to increase the area of targeted stimulation. Interferential current allows improved directional control and depth of penetration in comparison to other stimulation techniques.

Briefly described, in architecture, one embodiment of the invention, among others, can be implemented as follows.

Digital signal processors (DSPs) are used for improving the accuracy and reliability of digital signals that are used extensively in the communications field. Digital signal processing works by standardizing or clarifying the output of a digital signal. In this embodiment, the digital signal processor is used to shape multiple pulsatile waveforms to approximate the output of a sine-wave generator. In another embodiment of the invention, the digital signal processor is replaced with a field-programmable gate array (FPGA). An FPGA is an integrated circuit that can be programmed in the field after it is manufactured and therefore allows users to adjust the circuit output as the needs change. Both the DSP and the FPGA process a digital signal into a pseudo-sine-wave current waveform from the digital pulses generated by a pulse generator. The pseudo-sine-wave current waveform is transmitted through implantable quadripolar leads with eight electrodes at a targeted area creating a pair of interferential currents.

Other systems, methods, features, and advantages of the present invention will be or become apparent to one with skill in the art upon examination of the following drawings and detailed description. It is intended that all such additional systems, methods, features, and advantages be included within this description, be within the scope of the present invention, and be protected by the accompanying claims.

BRIEF DESCRIPTION OF THE DRAWINGS

Many aspects of the invention can be better understood with reference to the following drawings. The components in the drawings are not necessarily to scale, emphasis instead being placed upon clearly illustrating the principles of the present invention. Moreover, in the drawings, like reference numerals designate corresponding parts throughout the several views.

FIG. 1 is a perspective view of an interferential current set up by two circuits that are arranged in a cross pattern;

FIG. 2 is a perspective view of an interferential current pattern indicating the current intensity level and area of beat frequency formation;

FIG. 3 is a perspective view illustrating the effective area of stimulation resulting from the crossing of separate circuits; and

FIG. 4 is a diagram illustrating interferential stimulation using two implantable quadripolar leads.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

A preferred embodiment of the invention and modifications thereof will now be described with reference to the drawings.

FIG. 1 shows a stimulator 100 for the electrical stimulation of the spinal cord utilizing an interferential current 110 that has a base medium frequency alternating current within the range of 500 Hz-20 KHz. The interferential current 110 is set up between two circuits 118, 120 that are arranged in a cross-pattern. A first pair of implantable electrodes 108, 208 are positioned on a subject's spinal column 112, preferably the dorsal column, at one set of diagonal corners of a targeted area 214 (see FIG. 2). A second pair of implantable electrodes 108, 208 is then positioned at the other set of diagonal corners of the targeted area 214. Preferably, the electrodes 108 are attached to the dura matter in the epidural space. A digital signal processor 102 is connected to the first and second pairs of surface electrodes 108. When a signal generating source 104 is connected to the digital signal processor 102, a sine-wave-like waveform signal output 106 is created. The digital signal processor 102 improves the accuracy and reliability of digital signals. The digital signal processor 102 processes the multiple pulses 116 from the signal generating source 104 to approximate a sine-wave (pseudo-sine-wave or sine-wave-like). Thus, that type of current recruits larger numbers of dorsal column fibers and provides greater levels of pain relief.

The digital signal processor 102 generates individual pulses 106 of differing widths and resultant amplitudes. Preferably, the pulse width is set at 210 microseconds, but can range from 50-600 microseconds. When those differing pulses 106 are driven into a transformer (not shown), the pseudo-sine-wave is produced. A pulse generator 104 is connected to the digital signal processor 102 and supplies a pulsed digital signal output 116 to the digital signal processor 102. The digital signal 106 processed by the digital signal processor 102 creates a first circuit 118 and a second circuit 120 at the first and second pairs of surface electrodes 108, 208, respectively. Preferably, the range of output of the electrical circuits 118, 120 are 0-11 volts per circuit, depending on the patient's needs for pain treatment. Where the first and second circuits 118, 120 superimpose (cross), the resultant beat frequency (which is preferably between 1 and 250 beats/second) will be the difference between the frequencies of the two circuits, and the amplitude will be additive and greater than either circuit alone (FIG. 2).

Multiple target areas of the spinal cord can be treated depending upon the quantity and placement of the first and second pairs of electrodes 308, and by modulating the amplitudes of the outputs of the first and second circuits 318, 320 (see FIG. 3). Modulating the outputs of the first and second circuits 318, 320 increases the area of the targeted stimulation. The depth of modulation can vary from 0 to 100% and depends on the direction of the currents established by the first and second circuits 318, 320. It has been shown that when the first and second circuits 318, 320 intersect at 90°, the maximum resultant amplitude and the deepest level of modulation is half-way between the two circuits (45° diagonally). (See FIG. 2). Hence, the target area of stimulation can be augmented by modulation of the amplitudes of the outputs of the two circuits.

FIG. 4 illustrates two interferential currents 406 with sine-wave-like waveforms that are produced by two implantable quadripolar leads 409. Each quadripolar lead 409 includes four electrodes 408 for a total of eight. The two quadripolar leads 409 allow a greater target treatment stimulation area of the spinal cord. However, the invention could also apply to the use of two bipolar or octapolar lead systems, and other suitable devices. The electrodes could be activated in various combinations and patterns, and not just as shown in the drawings.

A field-programmable gate array (not shown) can also be used to shape multiple pulsatile waveforms to approximate the output of a sine-wave generator instead of the digital signal processor 102 described above. The FPGA is an integrated circuit that can be programmed in the field after it is manufactured and allows its user to adjust the circuit output as desired. In an alternative embodiment, the digital signal processor may be replaced with the FPGA. Whereas DSP processors typically have only eight dedicated multipliers at their disposal, a higher end FPGA device can offer up to 224 dedicated multipliers plus additional logic element-based multipliers as needed. That allows for complex digital signal processing applications such as finite impulse response filters, forward error correction, modulation-demodulation, encryption and applications such as utilized in the present invention.

It should be emphasized that the above-described embodiments of the present invention, particularly, any “preferred” embodiments, are merely possible examples of implementations, merely set forth for a clear understanding on the principles of the invention. Many variations and modifications may be made to the above-described embodiment(s) of the invention without departing substantially from the spirit and principles of the invention. All such modifications and variations are intended to be included herein within the scope of this disclosure and the present invention and protected by the following claims. 

We claim:
 1. An electrical stimulator for the treatment of intractable pain syndromes, comprising: an interferential current generator comprising a sine wave generator which generates an interferential alternating current output comprising first and second sinusoidal signals having different first and second frequencies with a base medium frequency of at least 500 Hz but no more than 20 KHz; and at least two pairs of implantable electrodes wherein each electrode has a first and a second end, wherein the first ends are connected to said interferential current generator and the second ends are configured to be implanted to a dura matter in an epidural space at predetermined locations proximate to a subject's spinal cord, wherein each pair of said at least two pairs of implantable electrodes transmits one of said first and second sinusoidal signals such that the first and second frequencies interfere with each other to produce at least one beat frequency signal proximate to the subject's spinal cord, and wherein a majority of the at least one beat frequency signal is directionally distributed and controlled, enabling the at least one beat frequency signal to avoid remaining in and shunting through cerebrospinal fluid proximate to the subject's spinal cord, thereby recruiting dorsal column fibers.
 2. The stimulator of claim 1, wherein said interferential current generator comprises: a pulse generator that generates digital signal pulses; and a digital signal processor connected to said pulse generator that processes the digital signal pulses to approximate a sine-wave-like output waveform.
 3. The stimulator of claim 1, wherein said interferential current generator comprises: a pulse generator that generates digital signal pulses; and a field-programmable gate array connected to said pulse generator that processes the digital signal pulses to approximate a sine-wave-like output waveform.
 4. The stimulator of claim 1, wherein said interferential current includes a resultant beat frequency of no more than 250 Hz.
 5. The stimulator of claim 1, wherein said interferential current includes a voltage output of 11 volts maximum for each circuit.
 6. The stimulator of claim 1, wherein said interferential current includes a pulse width with a range of at least 10 microseconds but no more than 600 microseconds.
 7. The stimulator of claim 1, wherein the at least two pairs of implantable electrodes comprise two quadripolar leads used to produce two interferential currents.
 8. An electrical stimulator for the treatment of intractable pain syndromes, comprising: an interferential current generator that generates an interferential alternating current output including first and second sinusoidal signals having different first and second frequencies, with a base medium frequency of at least 500 Hz but no more than 20 KHz; and at least two pairs of implantable electrodes having first and second ends, wherein the first ends are connected to said interferential current generator and the second ends are configured to be implanted to a dura matter in an epidural space at predetermined locations proximate to a subject's dorsal column, and wherein each of said at least two pairs of implantable electrodes carries one of said first and second sinusoidal signals such that the first and second frequencies interfere with each other to produce at least one beat frequency signal proximate to the subject's dorsal column, and wherein a majority of the at least one beat frequency signal is directionally distributed and controlled, enabling the at least one beat frequency signal to avoid remaining in and shunting through cerebrospinal fluid proximate to the subject's dorsal column, thereby recruiting dorsal column fibers.
 9. The stimulator of claim 8, wherein said interferential current generator comprises: a pulse generator that generates digital signal pulses; and a digital signal processor connected to said pulse generator that processes the digital signal pulses to approximate a sine-wave-like output waveform.
 10. The stimulator of claim 8, wherein said interferential current generator comprises: a pulse generator that generates digital signal pulses; and a field-programmable gate array connected to said pulse generator that processes the digital signal pulses to approximate a sine-wave-like output waveform.
 11. The stimulator of claim 8, wherein said interferential current includes a resultant beat frequency of no more than 250 Hz.
 12. The stimulator of claim 8, wherein said interferential current includes a voltage output of 11 volts maximum for each circuit.
 13. The stimulator of claim 8, wherein said interferential current includes a pulse width with a range of at least 10 microseconds but no more than 600 microseconds.
 14. The stimulator of claim 8, wherein the at least two pairs of implantable electrodes comprise two quadripolar leads used to produce two interferential currents. 